Usually large valves are closed by a spring force which is supplied from a spring which is maintained in compression, and which exerts a force on an actuating rod which is connected with or integral with the valve stem. The valve is opened by air under pressure which forces a piston connected to the actuating rod against the bias of the spring in the direction necessary to open the valve. Thus, if air pressure should drop, the valve would close under the action of the spring to give the system a fail-safe feature. Many large valves must be provided with a fail-safe closing system which will close the valve in the event of a catastrophe such as an earthquake or serious fire.
An example of a large valve which must be provided with a system for exerting a large closing force, and for maintaining the valve closed, is a large valve used in a main steam line between the steam generator and turbines in a large power plant. In nuclear power plants it is required that main steam isolation valves be capable of shutting safely when the air lines to the valve actuator are fractured as by an earthquake. This requirement is not easily met because the valves must overcome the high pressure in the steam line while being closed in spite of the fact that the air lines to the valves have been fractured, and the source of energy used to close the valves has been impaired.